Astrophysicists puzzle over intergalactic coincidence

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Astrophysicists puzzle
over intergalactic coincidence

Gamma-ray burst and supernova may
have no relation

Jan.
15, 1999: Coincidences are rare in nature, so when one occurs,
the natural reaction is to see if the two events are linked by
more than just chance, and if that link applies elsewhere in
the universe. Thus, some astrophysicists started to rethink the
non-linkage between gamma-ray bursts and supernovae after an
event in April 1998.

So far, says Dr. Marc Kippen, the odds are against gamma-ray
bursts being associated with supernovas.

"They might be related," he said, "but then
you must explain how a local supernova produced a gamma ray burst
that looks like all the other ones that evidently come from very
great distances."

Kippen, an astrophysicist with the University of Alabama in
Huntsville working at NASA's Marshall Space Flight Center, published
the results of his analysis in the Oct. 10 1998 issue of The
Astrophysical Journal.

"We can almost conclusively say that no bright gamma-ray
burst detected so far comes from a known supernova," Kippen
said. "We are less certain about weaker bursts because they
can't be precisely located. In addition, we miss most supernovas,
so about 10 percent of the weaker gamma-ray bursts could come
from supernovas."

Gamma-ray bursts have been one of the most mysterious phenomena
in the universe since their discovery about 30 years ago. Initially
they were thought to be associated with neutron stars within
our galaxy. Observations with the Burst and Transient Source
Experiment (BATSE) board the Compton Gamma Ray Observatory, launched
in 1991, have shifted the scene from our galaxy to deep in the
universe.

BATSE scientists found that gamma ray bursts are randomly
distributed across the sky, indicating that they are peppered
throughout the heavens, rather than clustered along the plane
of our Milky Way galaxy, which would be expected if they were
part of the galaxy.

Since 1997, a few bursts have been observed with optical,
X-ray, or radio counterparts that are thought to be at cosmological
distances up to 10 to 12 billion light years away. Thus, the
original explosion had to be incredibly powerful to produce a
flash of gamma radiation that is readily detected at Earth.

Then came a burst and a supernova both on April 25, 1998,
both in the same region of the sky. The gamma-ray burst was observed
by BATSE and the Beppo SAX satellite. The precise position of
the burst provided by instruments on Beppo SAX allowed ground-based
optical telescopes to discover that the burst was coincident
with a new supernova - SN1998bw - within the same small section
of the sky.

This presented a new challenge because the gamma-ray burst
was average in its properties. Nothing distinguished it from
the other bursts routinely detected by BATSE. However, sn1998bw
was extraordinary - the intrinsically brightest supernova ever
observed in its category.

It could be a coincidence. BATSE has recorded more than 2,000
bursts since 1991, about one a day. And only 1 supernova in 10
is actually detected. So, a burst and a supernova are bound to
coincide in time and apparent location.

But if they're related, it could
send astrophysicists back to rethink the mystery.

"I looked to see if there were any more of these coincidences,"
Kippen said. "The chances of observing a supernova within
a small bit of sky over the span of a few days is pretty small,
about 1 in 10,000." Kippen divided the BATSE burst catalog
into two groups. First he looked for bursts that were seen both
by BATSE and Ulysses. BATSE has eight detector modules. Measuring
the brightness of a burst as seen by the three or four modules
that are actually triggered will describe a large error box in
the sky. Triangulating the arrival time of the burst with the
time of arrival at Ulysses, located deep within our solar system,
reduces the error box to a short, thin arc across the sky.

Left: The Compton Gamma Ray Observatory
is deployed by the Space Shuttle in April 1991. Four of the eight
BATSE instruments can be seen at the spacecraft's corners. The
other four are on the opposite face. (NASA)

Kippen then compared these with supernovas that had been detected
at about the same time as the burst. He allowed a generous margin
- up to a month - since the dates of supernova explosions aren't
always precisely known.

He came up empty handed. The 415 bursts and 585 supernovas
all had separate locations.

Still, there is the possibility that the supernovas might
cause weaker bursts that were detected by BATSE and not by Ulysses
which carries a much smaller instrument. This gave him a set
of 1,222 bursts.

"At some level you expect gamma rays to come from a supernova,"
Kippen said. But they should be less powerful than the events
that cause gamma-ray bursts at cosmological distances, and their
light profiles - how they brighten and dim - also should be different.

"The result for using just BATSE locations was that there's
no significant excess," Kippen said. "It was consistent
with random locations. "

A plot of 160 recent supernova
locations (red diamonds) shows that none coincides with 415 gamma-ray
bursts seen by both BATSE and a detector on the Ulysses international
solar polar spacecraft (purple lines). The supernova locations
are shown in diamonds for clarity; their actual locations are
known to much greater precision. The BATSE/Ulysses lines are
the overlap between a great circle formed by timing triangulation
between BATSE and Ulysses, and the rough position defined by
BATSE alone. this plot also depicts the random nature of gamma-ray
bursts. Because this sample covers 1991-97, SN1998bw is not shown.
All of these sources are outside our galaxy. For reference, the
Milky Way lies along the equator of this map. Links to

The issue likely will remain open for some time, until the
right combination of instruments happens to be pointing in the
direction of a supernova that also yields a burst, or until more
advanced telescopes are built.

"The next-generation gamma-ray detectors could have the
capability of observing hundreds of bursts per year with very
accurate positions," Kippen said. "Then you could start
to make more definitive statements. Then we could also precisely
localize weak bursts and say whether any of them occur with supernovas."

In the meantime, astrophysicists are left with three possibilities.

"The first is that this is just a random coincidence,"
Kippen said.

The second is that the burst and supernova are related. "Then
you have to explain why a supernova produced burst that looked
just like all the other ones seen at presumably very large distances,"
he continued.

The third possibility is that most bursts occur at great cosmological
distances, but a small percentage - probably less than 6 percent
- are produced by a different mechanism in our galactic neighborhood.

For now, scientists may have to live with an uncomfortable
coincidence.

"One event is just one event," Kippen said. "It's
not overwhelming evidence."